Display driver circuits

ABSTRACT

Display driver circuits are described for driving organic light emitting diode displays, particularly passive matrix displays, with greater efficiency. The display drivers comprise a controllable current generator to provide a variable current drive output to an OLED display, the current generator comprising at least one bipolar transistor in series with the current drive output. The display brightness is adjustable by controlling the current generator to vary the current drive to the display. Preferably the bipolar transistor has an emitter terminal substantially directly connected to a power supply line of the driver to reduce losses in the driver. A corresponding method is also described. By employing a bipolar transistor current drive and varying display brightness by controlling the current an efficient driver-display combination is obtained.

This invention generally relates to display driver circuits forelectro-optic displays, and more particularly relates to circuits andmethods for driving organic light emitting diode displays, especiallypassive matrix displays, with greater efficiency.

Organic light emitting diodes (OLEDs) comprise a particularlyadvantageous form of electro-optic display. They are bright, colorful,fast-switching, provide a wide viewing angle and are easy and cheap tofabricate on a variety of substrates. Organic LEDs may be fabricatedusing either polymers or small molecules in a range of colours (or inmulti-coloured displays), depending upon the materials used. Examples ofpolymer-based organic LEDs are described in WO 90/13148, WO 95106400 andWO 99/48160, examples of so called small molecule based devices aredescribed in U.S. Pat. No. 4,539.507.

A basic structure 100 of a typical organic LED is shown in FIG. 1 a. Aglass or plastic substrate 102 supports a transparent anode layer 104comprising, for example, indium tin oxide (TTO) on which is deposited ahole transport layer 106, an electroluminescent layer 108, and a cathode110. The electroluminescent layer 108 may comprise, for example, a PPVpoly(p-phenylenevinylene)) and the hole transport layer 106, which helpsmatch the hole energy levels of the anode layer 104 andelectroluminescent layer 108, may comprise, for example, PEDOT:PSS(polystyrene-sulphonate-doped polyethylene-dioxythiophene). Cathodelayer 110 typically comprises a low work function metal such as calciumand may include an additional layer immediately adjacentelectroluminescent layer 108, such as a layer of aluminum, for improvedelectron energy level matching. Contact wires 114 and 116 to the anodethe cathode respectively provide a connection to a power source 118. Thesame basic structure may also be employed for small molecule devices.

In the example shown in FIG. 1 a light 120 is emitted throughtransparent anode 104 and substrate 102 and such devices 3 referred toas “bottom emitter”. Devices which emit through the cathode may also beconstructed, for example by keeping the thickness of cathode layer 110less than around 50-100 nm so that the cathode is substantiallytransparent.

Organic LEDs may be deposited on a substrate in a matrix of pixels toform a single or multi-colour pixellated display. A multicoloureddisplay may be constructed using groups of red, green and blue emittingpixels. In such displays the individual elements are generally addressedby activating row (or column) lines to select the pixels, and rows (orcolumns) of pixels are written to, to create a display. So-called activematrix displays have a memory element, typically a storage capacitor anda transistor, associated with each pixel whilst passive matrix displayshave no such memory element and instead are repetitively scanned,somewhat similarly to a TV picture, to give the impression of a steadyimage.

FIG. 1 b shows a cross section trough a passive matrix OLED display 150in which like elements to those of FIG. 1 a are indicated by likereference numerals. In the passive matrix display 150 theelectroluminescent layer 108 comprises a plurality of pixels 152 and thecathode layer 110 comprises a plurality of mutually electricallyinsulated conductive lines 154, running into the page in FIG. 1 b, eachwith an associated contact 156. Likewise the ITO anode layer 104 alsocomprises a plurality of anode lies 158, of which only one is shown inFIG. 1 b, running at right angles to the cathode lies. Contacts (notshown in FIG. 1 b) are also provided for each anode line. Anelectroluminescent pixel 152 at the intersection of a cathode line andanode line may be addressed by applying a voltage between the relevantanode and cathode lines.

Referring now to FIG. 2 a, this shows, conceptually, a drivingarrangement for a passive matrix OLED display 150 of the type shown inFIG. 1 b. A plurality of constant curt generators 200 are provided, eachconnected to a supply line 202 and to one of a plurality of column lines204, of which for clarity only one is shown. A plurality of row lines206 (of which only one is shown) is also provided and each of these maybe selectively connected to a ground line 208 by a switched connection210. As shown, with a positive supply voltage on line 202, column lines204 comprise anode connections 158 and row lines 206 comprise cathodeconnections 154, although the connections would be reversed if the powersupply line 202 was negative and with respect to ground line 208.

As illustrated pixel 212 of the display has power applied to it and istherefore illuminated. To create an image connection 210 for a row ismaintained as each of the column lines is activated in turn until thecomplete row has been addressed, and then the next row is selected andthe process repeated. Alternately a row may be selected and all thecolumns written in parallel, that is a row selected and a current drivenonto each of the column lines simultaneously, to simultaneouslyilluminate each pixel in a row at its desired brightness. Although thislatter arrangement requires more column drive circuitry it is preferredbecause it allows a more rapid refresh of each pixel. In a furtheralternative arrangement each pixel in a column may be addressed in turnbefore the next column is addressed, although this is not preferredbecause of the effect, inter alia, of column capacitance as discussedbelow. It will be appreciated that in the arrangement of FIG. 2 a thefunctions of the column driver circuitry and row driver circuitry may beexchanged.

It is usual to provide a current-controlled rather than avoltage-controlled drive to an OLED because the brightness of an OLED isdetermined by the current flowing through it, this determining thenumber of photons it outputs. In a voltage-controlled configuration thebrightness can vary across the area of a display and with time,temperature, and age, making it difficult to predict how bright a pixelwill appear when driven by a given voltage. In a colour display theaccuracy of colour representations may also be affected.

FIGS. 2 b to 2 d illustrate, respectively, the current drive 220 appliedto a pixel, the voltage 222 across the pixel, and the light output 224from the pixel over time 226 as the pixel is addressed. The row coiningthe pixel is addressed and at the time indicated by dashed line 228 thecurrent is driven onto the column line for the pixel. The column line(and pixel) has an associated capacitance and thus the voltage graduallyrises to a maximum 230. The pixel does not begin to emit light until apoint 232 is reached where the voltage across the pixel is greater thanthe OLED diode voltage drop. Similarly when the drive current is tamedoff at time 234 the voltage and light output gradually decay as thecolumn capacitance discharges. Where the pixels in a row are all writtensimultaneously, that is where the column are driven in parallel, thetime interval between times 228 and 234 corresponds to a line scanperiod.

It is desirable to be able to provide a greyscale-type display, that isone in which the apparent brightness of individual pixels maybe variedrather than simply set either on or off. In the context of thisinvention “greyscale” refers to such a variable brightness display,whether a pixel is white or coloured.

The conventional method of varying pixel brightness is to vary pixelon-time using Pulse Width Modulation (PWM). In the context of FIG. 2 babove the apparent pixel brightness may be varied by varying thepercentage of the interval between times 228 and 234 for which drivecurrent is applied. In a PWM scheme a pixel is either full on orcompletely off but the apparent brightness of a pixel varies because ofintegration within the observers eye.

Pulse Width Modulation schemes provide a good linear brightness responsebut to overcome effects related to the delayed pixel turn-on theygenerally employ a precharge current pulse (not shown in FIG. 2 b) onthe leading edge 236 of the driving current waveform, and sometimes adischarge pulse on the trailing edge 238 of the waveform. As a result,charging (and discharging) the column capacitance can account for halfthe total power consumption in displays incorporating this type ofbrightness control. Other significant factors which the applicant hasidentified as contributing to the power consumption of a display plusdriver combination include dissipation within the OLED itself (afunction of OLED efficiency), resistive losses in the row and columnlines and, importantly in a practical circuit, the effects of a limitedcurrent driver compliance, as explained in more detail later.

FIG. 3 shows a schematic diagram 300 of a generic driver circuit for apassive matrix OLED display. The OLED display is indicated by dashedline 302 and comprises a plurality n of row lines 304 each with acorresponding row electrode contact 306 and a plurality m of columnlines 308 with a corresponding plurality of column electrode contacts310. An OLED is connected between each par of row and column lines with,in the illustrated arrangement, its anode connected to the column line.A y-diver 314 drives the column lines 308 with a constant current and anx-diver 316 drives the row lines 304, selectively connecting the rowlines to ground. The y-driver 314 and x-driver 316 are typically bothunder the control of a processor 318. A power supply 320 provides powerto the circuitry and, in particular, to y-driver 314.

Specific examples of OLED display drivers are described in U.S. Pat. No.6,014,119, U.S. Pat. No. 6,201,520, U.S. Pat. No. 6,332,661, EP1,079,361A and EP 1,091,339A; OLED display driver integrated circuitsare also sold by Clare Micronix of Clare, inc., Beverly, Mass., USA. TheClare Micronix drivers provide a current controlled drive and achievegreyscaling using a conventional PWM approach.

U.S. Pat. No. 6,014,119 describes a driver circuit in which pulse widthmodulation is used to control brightness and in which the emitters ofdriver transistors are conned to a fixed voltage via resistors, theresistors being considered to approximate a substantially ideal currentsource.

U.S. Pat. No. 6,201,520 describes driver circuitry in which each columndriver has a constant current generator associated with a set ofswitches which allows non selected pixels to be reverse biased toprevent crosstalk. The described circuits provide digital (on/off) pixelcontrol but are not suitable for displays in which the brightness ofindividual pixels need to be independently adjustable, for exampleanalogue, greyscale-type displays.

U.S. Pat. No. 6,332,661 describes pixel driver circuitry in which areference current generator sets the current output of a constantcurrent driver for a plurality of columns, but again this arrangement isnot suitable for analogue greyscale type displays.

EP 1,079,361A and EP 1,091,339A both describe similar drivers fororganic electroluminescent display elements in which a drive voltage(rather than a drive current) for an organic EL element is set by amicrocomputer driving an operational amplifier, negative feedback beingapplied in an attempt to compensate for characteristic variations andageing effects associated with voltage drive circuits.

It is generally desirable to reduce the power consumption of the displayplus driver combination, especially whilst retaining the ability toprovide a greyscale display. It is further desirable to reduce themaximum required power supply voltage for the display plus drivercombination.

According to the present invention there is therefore provided a displaydriver for a passive organic electroluminescent display, the displaydriver comprising a controllable current generator to provide a variablecurrent drive output to the display, the current generator comprising atleast one bipolar transistor in series with the current drive outputwhereby the display brightness is adjustable by controlling the currentgenerator to vary the current drive to the display.

A current source attempts to deliver a substantially constant current tothe load to which it is connected but it will be appreciated that therewill come a point as its output voltage approaches the supply voltage,at which this is no longer possible. The range of voltages over which acurrent source provides an approximately constant current to a load istermed the compliance of the current source. The compliance can becharacterised by (V_(s)-V_(o)) where V_(s) is the supply voltage andV_(o) is substantially the maximum output voltage of the current sourcein that when V_(s)-V_(o) is small the compliance is high, andvice-versa. (For convenience in this specification reference will bemade to a current source and to current sources but these may besubstituted by a current sins or sinks).

The applicants have recognised that the lower the current drivercompliance (i.e. the greater V_(s)-V_(o)), the greater the power lossesdue to limited driver compliance. The lower the driver circuitcompliance the greater the supply voltage to the current driver must bein order to obtain a maximum desired pixel brightness, as can be seen byan example. Consider an OLED driver in which the pixel brightness iscontrolled by controlling the drive current I_(drive) rather than bypulse width modulating the OLED. Assume I_(drive) for maximum brightnesscan be obtained with a voltage across the OLD of approximately S voltsand consider the two cases, where the current generator requires asupply voltage of 9 volts and where the current generator requires asupply voltage of 12 volts to provide the required maximum I_(drive).When the, OLED is half on the voltage across it will be approximately 4volts (for the sake of this example, although in practice it will be nonlinear). It can therefore be see that the power loss in the two cases(ΔV, I_(drive)) in the two cases is 5 I_(drive) and 8 I_(drive)respectively—in other words almost double the loss for the lowcompliance (large V_(s)-V_(o)) current generator.

The applicants have recognised that the power losses in the currentgenerator can be substantially reduced by designing a (variable) currentgenerator to have a high compliance, that is to have a low value ofV_(s)-V_(o). This is done by employing a bipolar transistor to improvethe compliance of the current generator.

Preferably the display is a passive matrix OLED display and the driveris configured such that the brightnesses of individual pixels in a rowor column can be substantially independently adjusted in order toprovide a greyscale image. Thus the display driver preferably has aplurality of current generators for simultaneously driving a pluralityof either or row or column electrodes so that, for example, all thepixels in a row may be driven simultaneously. This helps increase theframe rate and thus the apparent brightness of the display, as well asreducing flickering.

Preferably the bipolar transistor has an emitter terminal substantiallydirectly connected to a power supply line of the driver. This does notnecessarily require that the emitter terminal should be connected to apower supply line or terminal for the driver by the most direct routebut rather at there should be no intervening components (apart from theinstinct resistance of tracks or connections within the drivercircuitry) between the emitter and a power supply rail. In this way thepower supply voltage can be kept to a minimum necessary to drive thedisplay at its maximum desired brightness. Preferably the voltage dropbetween the emitter terminal and the power supply line is less thanexpected statistical variations in V_(be) of the transistor, that istypically less than 100 mV, probably less than 50 mV.

Preferably the controllable current genorator comprises a current mirroras this allows V_(o) to approach, typically to within less than 0.5V ofthe supply, and sometimes to within 0.1V of the supply. A pair ofbipolar transistors need not be provided for each driver circuit(although his may be preferable in some embodiments) as a current mirrorcircuit may, in effect, be shared by a plurality of driver circuits, forexample across a plurality of display column electrodes. A currentmirror has a finite output impedance and thus the output current canvary by up to 25% over the output compliance range (broadly becauseV_(be) varies slightly with collector voltage for a given drivecurrent). This effect can be reduced by employing a Wilson currentmirror although the compliance is then degraded.

Preferably the display driver includes control circuitry for the currentgenerator using a different transistor technology, preferably thesmaller, lower power MOS (Metal Oxide Semiconductor) technology. In thisway the total power consumption of the driver may be further reduced. Ina preferred embodiment the control circuitry includes a digital toanalogue converter to provide an analogue current output to the currentgenerator, for example a variable current output to programme the loadcurrent provided to the display by a current mirror circuit.

In another aspect the invention provides display driver circuitry for anorganic electroluminescent display, the display comprising a pluralityof organic electroluminescent pixels each pixel being addressed by oneof each of a first plurality and a second plurality of drivingelectrodes, the display driver circuitry comprising, a power supplyconnection, a plurality of electrode driver outputs for driving aplurality of said display driving electrodes, a plurality of variablecurrent driver circuits, each having a control input and providing avariable current drive for a said electrode driver output, and eachcomprising a bipolar current drive transistor having an emitter terminalsubstantially directly connected to said power supply connection and acollector terminal coupled to a said electrode driver output; andcontrol circuitry coupled to the control inputs of said driver circuitsand configured to provide an analogue signal to each said control inputfor controlling the variable current drive provided by each said drivercircuit, whereby the brightness of each pixel is adjustable.

Again by providing a variable or analogue current drive using a bipolarcurrent drive transistor with an emitter terminal substantially directlyconnected to a power supply connection or bus for the circuit, that iswithout intervening components, a power-efficient variable brightnessOLED pixel display driver maybe provided. It will be appreciated thatalthough the control input signal is analogue it may be quantised, forexample where the control circuitry is itself controlled by a digitalsignal.

The collector terminal of the drive transistor is preferably alsosubstantially directly connected to the electrode driver output fordriving a display, thus helping to minimise unwanted voltage drops andto increase the overall operating efficiency of the driver plus displaycombination. The collector terminal may be connected to the load via anintervening component or components (such as another bipolar transistor,for example when using a Wilson current mirror) but this is notpreferred because the compliance is reduced.

The display driver circuitry is suitable for either small molecule orpolymer LEDs and is especially suitable for providing greyscale imageson passive matrix OLED displays, that is displays in which differentpixels may need to be set at different brightnesses.

Preferably the current driver circuit comprises a current mirror,preferably two bipolar transistors being provided for each column (orrow) of the display. The bipolar current drive transistor may comprise aDarlington transistor or a variant such as a complementary Darlington(also known as a complementary feed-back pair or Sziklai connectedpair). Preferably the control circuitry consists of MOS transistors, fora further power saving. In one embodiment the control circuitrycomprises a plurality of FET switches each coupled to a respectivecurrent setting component for example a current setting resistor.

In a third aspect the invention provides a column electrode displaydriver for a passive OLED display, the display comprising a matrix ofOLEDs and a plurality of row and column electrodes, each OLED having ananode coupled to a said column electrode and a cathode coupled to a saidrow electrode, the display driver comprising a plurality ofindependently adjustable, high compliance bipolar column current driverseach having an analogue input for providing a variable column cent driveto the display.

In a further aspect the invention provides display driver circuitry foran organic electroluminescent display, the display comprising aplurality of organic electroluminescent pixels each pixel beingaddressed by one of each of a first plurality and a second plurality ofdriving electrodes, the display driver circuitry comprising, a powersupply connection, a plurality of electrode driver outputs for driving aplurality of said display driving electrodes, a plurality of drivercircuits for each driving electrode for providing a plurality of currentdrives for a said electrode driver output, each having a control inputand each comprising a bipolar current drive transistor having an emitterterminal substantially directly connected to said power supplyconnection and a collector terminal coupled to a said electrode driveroutput; and control circuitry coupled to the control inputs of saiddriver circuits and configured to provide a signal to each said controlinput for controlling the current drive provided to a said electrodedriver output, whereby the brightness of each pixel is adjustable.

The invention also provides a corresponding method of increasing theefficiency of a current driver for an organic electroluminescentdisplay, the method comprising, using a bipolar transistor with anemitter terminal substantially directly connected to a power line toprovide a current drive for the display; and using a voltage for thepower line such that when the organic electroluminescent display is atsubstantially a maximum desired brightness the bipolar transistorcurrent drive is operating substantially at its limit of compliance.

As described above, excess power dissipation in an OLED current drivercan be reduced by employing a bipolar transistor substantially directlyconnected to a power line of the driver and then by choosing a powersupply voltage which is no more than necessary to give the requireddisplay brightness when the current drive is operating substantially atits limit of compliance, that is when the output voltage from thecurrent drive is substantially the maximum possible at the selectedpower supply voltage for the current drive. Preferably the display avariable brightness display and the current drive is a variable currentdrive as this allows further power savings as compared with conventionalPWM brightness control.

These and other aspects of the invention will now be further described,by way of example only with reference to the accompanying figures inwhich:

FIGS. 1 a and 1 b show cross sections through, respectively, an organiclight emitting diode and a passive matrix OLED display;

FIGS. 2 a to 2 d show, respectively, a conceptual driver arrangement fora passive matrix OLED display, a graph of current drive against time fora display pixel, a graph of pixel voltage against time, and a graph ofpixel light out against time;

FIG. 3 shows a schematic diagram of a generic driver circuit for apassive matrix OLED display according to the prior art;

FIG. 4 shows a block diagram of a passive matrix OLED display pixeldriver according to an embodiment of the present invention;

FIG. 5 shows a schematic diagram of a passive matrix OLED display drivercircuit according to an embodiment of the present invention; and

FIGS. 6 a to 6 c show, respectively, a bipolar Darlington transistordriver, a bipolar complementary Darlington transistor driver, and abipolar current mirror driver with multiple outputs.

FIG. 4 shows a block diagram of a generic variable current bipolartransistor driver 400 for an organic electroluminescent display. Thedriver comprises a bipolar current driver portion 406 and a MOScontroller portion 410; typically these will both be integrated on acommon integrated circuit substrate. A power supply 402, external to theintegrated circuit is connected to a power line 404 within theintegrated circuit and to a ground connection 403 of the integratedcircuit.

The bipolar current driver 406 has a current control input 410 andprovides a controlled current output 408 to the display, in the case ofa passive matrix display to a column or row line. The current controlinput 410 allows the current on column line 408 to be varied betweenminimum and maximum current outputs corresponding to minimum and maximumdesired display brightnesses.

The control signal on input 410 is preferably an analogue control signalprovided from MOS controller 410 in response to a dial brightnesscontrol signal 412 provided to the MOS controller 410. Thus preferablythe MOS controller 410 comprises a digital to analogue converter. Itwill be appreciated that although the current driver 406 is in principlecapable of outputting a continuously variable current, in practice thedigital control signal constrains the output current to one of a numberof discreet values determined by the number of bits in the digitalcontrol signal.

The bipolar current driver 406 is designed to allow the output voltageon line 408 to approach as close as possible to the supply voltage onpower line 404, thus giving the driver a high compliance. This isfacilitated by directly connecting an emitter terminal of a bipolarcurrent driver transistor (i.e. a transistor in series with the columnline connection) substantially directly to power line 404. Preferablythe current driver 406 also includes a bipolar transistor driver with acollector terminal directly connected to display drive line 408. Thisallows the output voltage of the current driver to closely approach thesupply voltage, thus increasing the overall power efficiency of thedisplay plus driver combination in operation.

Referring now to FIG. 5, this shows a schematic diagram of displaydriver circuitry for a passive matrix OLED display 502. The display 502is similar to the display 302 of FIG. 3, although for convenience onlyfour pixels are shown. Like elements of display 502 to those of display302 have been given like reference numerals.

The driver circuitry comprises a MOS controller 504 a, b and a bipolarcurrent driver 506 a, b for each column line output 508 a, b. Likewise arow line driver 510 a, b is provided for each row line connection 512 a,b. In practice a driver integrated circuit will have many such rowand/or column drivers, and large passive matrix displays may requiremany such driver integrated circuits. Here, for convenience, only onecolumn line driver 504, 506 and only one row line driver 510 will bedescribed in detail as the others are similar.

The driver circuitry is powered an external power supply 516 connectedto power 518 and ground 520 line connections of the driver circuitry.The bipolar current driver 506 a comprises a pair of PNP bipolartransistors 522, 524 connected to power line 518 in a current mirrorconfiguration. In such a configuration a current on input line 526controls a load current on output 508 a, the ratio of the two currentsbeing determined by the ratio of the transistor junction areas (formatched transistors).

The MOS controller 504 a comprises three FET switches 528, 530, 532 eachconnected to a respective power supply 534, 536, 538. The gateconnections of the transistors 529, 531, 533 comprise a tree bit digitalinput each switching a respective power supply 534, 536, 538 to acorresponding current setting resistor 540, 542, 544. Each of resistors540, 542, 544 is connected to the current input 526 of the currentmirror 506 a. Each of the power supplies 534, 536, 538 has a voltagewhich is approximately twice that of the next lowest power supply (notexactly because of the V_(be) drop) so that a digital value on FET gateconnections 529, 531, 533 is converted into a corresponding current online 526 so that MOS controller 504 a converts a digital voltage valueto an analogue current. In other embodiments power supplies 534,536 and538 may have the same voltage and the value of resistors 540, 542, 544may be scaled in powers of two. Different voltage and/or resistor valuesmay be employed according to the application and other designconsiderations.

Transistor 524 is directly connected between power line 518 and columnline output 508 a (discounting any track resistance), thus maximisingthe compliance of the current driver or, in other words, allowing thevoltage on column line output 508 a to approach as close as possible tothat of power Line 518. This is in contrast to a conventional currentsource which employs an emitter resistor to stabilise the current outputand, in particular, to compensate for statistical variations in V_(be)on I_(o) in different transistors, With the design shown the voltage oncolumn line 508 a may approach within one volt, 500 mV or even to within100 mV of power line 518.

Where the site of transistors 522 and 524 is the same the current mirrorof driver 506 a provides a 1:1 ratio of input current on line 526 tooutput current on line 508 a (where the two transistors aresubstantially matched to one another). Power can be saved by reducingthe current in transistor 522, for example by scaling the currentmirror. The currents in lines 526 and 508 a are in proportion to theareas of transistors 522 and 524 (for matched transistors) and thus thecurrent in transistor 522 may be reduced by making it smaller, forexample {fraction (1/10)} or less, {fraction (1/30)} or less, or{fraction (1/50)} or less of the size of transistor 524. Alternativelythe current mirror may be scaled by fabricating a further transistorwith its base, emitter, and collector in parallel with transistor 524,for example to give a 1:2 scaling. Preferably the current in transistor522 is around 10% or less of the current in transistor 524.

The row drive circuitry essentially comprises a single bipolartransistor 510 a, b controlled by a voltage (or current) applied to itsrespective base connection 546 a, b. For the sake of illustration therow drivers are shown controlled by current sources 548 a, b although inpractice they will normally be driven by a processor such as processor318 of FIG. 3 via a suitable interface. The base current is thus drawnfrom the (lower voltage) logic supply.

Broadly speaking the row driver transistors 510 a, b simply act asswitches to connect a selected row line to ground. Again, however, it ispreferable that as little voltage as possible is dropped across thesetransistors, although each must handle a maximum current of nI_(col)where n is the number of columns and I_(col) is the maximum column drivecurrent.

Since transistors 510 are essentially only acting as switches to sinkcurrent to ground they can be driven hard, preferably into saturation.Preferably the transistors are also physically large to reduce theirresistance and hence increase the overall driver compliance. It is alsodesirable that the driver circuitry makes efficient use of silicon areaand for this reason bipolar transistors are preferred. However MOStransistors could be used instead of bipolar transistors as it ispractical to make these large enough to reduce the voltage drop to apractical value when the transistor is on. Typical maximum columncurrent values are between 0.1 mA and 1 mA and thus maximum row currentsmay be up to 100 mA or more.

FIG. 6 shows some of the variations which are possible to the circuit ofFIG. 5. Thus, for example, bipolar transistor 524 could be replaced bythe Darlington transistor configuration 600 of FIG. 6 a or alternativelythe complementary Darlington (or complementary feedback pair or Sziklai)configuration 610 of FIG. 6 b. These configurations provide higher gainthus reducing the base current required but both also introduce anadditional diode voltage drop between the emitter and collector and thusreduce the compliance. By increasing the gain of transistor 524 againthe current in transistor 522 is reduced thus providing a power saving.For example the current in transistor may be reduced to about {fraction(1/30)} or 3% of that in transistor 524.

In another alternative configuration as shown in FIG. 6 c a currentmirror circuit 620 is shared between a plurality of column lines using amultiple-collector transistor 622. If only one such current mirror 620is employed for a set of column lines the column current for each of thelines must be set in turn where different column currents are needed onthe different lines. However the current mirror 620 may be employed in avariant of the circuit of FIG. 5 in which the control circuitry selectsone of a predetermined set of currents for the column lines rather thancontrolling a bipolar current driver on each column line. Thus, forexample, where the display provides sixteen different levels of pixelbrightness (for bit control) sixteen current mirrors 620 may be providedto supply the sixteen different levels of current to each of the columnline outputs of the driver, the control circuitry then selecting anappropriate current. Where there are fewer levels of pixel brightnessthan columns this approach can simplify the driver circuitry.

In a fiber variant (not shown) the current mirror may be dispensed withand the base voltage of transistor 524 controlled to control the outputcurrent, although it is more difficult to obtain accurate currentcontrol with such an arrangement. The skilled person will alsoappreciate that although the circuit of FIG. 5 uses PNP transistors toprovide current sources, the circuit may also be inverted to employcontrollable current sink drivers using NPN transistors.

No doubt many other effective alternatives will occur to the skilledperson and it should be understood that the invention is not limited tothe described embodiments but encompasses modifications apparent tothose skilled in the art lying within the spirit and scope of the claimsappended hereto.

1. A display driver for a passive organic electroluminescent display,the display driver comprising: a controllable current generator toprovide a variable current drive output to the display, the currentgenerator comprising at least one bipolar transistor in series with thecurrent drive output; whereby the display brightness is adjustable bycontrolling the current generator to vary the current drive to thedisplay.
 2. A display driver as claimed in claim 1 wherein the passiveorganic electroluminescent display is a passive matrix display having aplurality of pixels addressed by row and column electrodes, the displaydriver having a plurality of said current generators for driving aplurality of ones of said row and column electrodes, whereby thebrightnesses of said pixels are adjustable to provide a greyscaledisplay.
 3. A display driver as claimed in claim 1 wherein said bipolartransistor has an emitter terminal substantially directly connected to apower supply line of the driver.
 4. A display driver as claimed in claim3 wherein the controllable current generator includes a current mirrorcircuit.
 5. A display driver as claimed in claim 1 further comprisingcontrol circuitry to control the current generator, the controlcircuitry comprising at least one method oxide semiconductor (MOS)transistor.
 6. A display driver as claimed in claim 1 further comprisingcontrol circuitry to control the current generator; wherein the currentgenerator is controllable by a current input, and wherein the controlcircuitry is configured to provide a variable current output to thecurrent generator current input.
 7. A display driver as claimed in claim6 wherein the control circuitry includes a digital-to-analog converterto convert a digital control signal input to an analog signal forproviding said variable current output.
 8. Display driver circuitry foran organic electroluminescent display, the display comprising aplurality of organic electroluminescent pixels, each pixel beingaddressed by one of each of a first plurality and a second plurality ofdriving electrodes, the display driver circuitry comprising: a powersupply connection; a plurality of electrode driver outputs for driving aplurality of said display driving electrodes; a plurality of variablecurrent driver circuits, each having a control input and providing avariable current drive for a said electrode driver output, and eachcomprising a bipolar current drive transistor having an emitter terminalsubstantially directly connected to said power supply connection and acollector terminal coupled to a said electrode driver output; andcontrol circuitry coupled to the control inputs of said driver circuitsand configured to provide an analog signal to each said control inputfor controlling the variable current drive provided by each said drivercircuit; whereby the brightness of each pixel is adjustable.
 9. Displaydriver circuitry as claimed in claim 8 wherein each said current drivercircuit comprises a current mirror having an output coupled to a saidelectrode driver output and a current control line coupled to saidcontrol input.
 10. Display driver circuitry as claimed in claim 8 Of 9wherein said bipolar current drive transistor comprises a Darlingtontransistor pair.
 11. Display driver circuitry as claimed in claim 8,wherein said control circuitry comprises method oxide semiconductor(MOS) transistors.
 12. Display driver circuitry as claimed in claim 11wherein a said current driver control input comprises a current setinput wherein the control circuitry for each of said current drivercircuits comprises a controllable current setting means coupled to saidcurrent set input.
 13. Display driver circuitry as claimed in claim 12wherein the controllable current setting means comprises a plurality offield effect transistor (FET) switches each coupled to a respectivecurrent setting component.
 14. Display driver circuitry as claimed inany one of claim 8 wherein said electrode driver outputs are configuredfor driving electrodes of one of said first and second pluralities ofdisplay driving electrodes, and further comprising a plurality of secondbipolar transistors for switching current from electrodes of the otherof said first and second pluralities of display driving electrodes. 15.Display driver circuitry as claimed in claim 14, further comprising asecond power supply connection, and wherein each said second bipolartransistor has an emitter terminal substantially directly connected tosaid second power supply connection and a collecter terminal coupled toa connection for an electrode of the other of said first and secondpluralities of display driving electrodes.
 16. A column electrodedisplay driver for a passive organic light-emitting diode (OLED)display, the display comprising a matrix of OLEDs and a plurality of rowand column electrodes, each OLED having an anode coupled to a columnelectrode and a cathode coupled to a row electrode, the display drivercomprising a plurality of independently adjustable, high compliancebipolar column current drivers each having an analog input for providinga variable column current drive to the display.
 17. A column electrodedisplay driver as claimed in claim 16 wherein each said column currentdriver comprises a bipolar transistor having an emitter directlyconnected to a power supply connection.
 18. A column electrode displaydriver as claimed in claim 17 further comprising a metal oxidesemiconductor (MOS) column current drive controller.
 19. A columnelectrode display driver as claimed in claim 18 wherein each said columncurrent driver comprises a current mirror with a current controlconnection, and wherein said MOS controller is configured to control acurrent in the current control connection of each column current driver.20. Display driver circuitry for an organic electrolmninescent display,the display comprising a plurality of organic electroluminescent pixels,each pixel being addressed by one of each of a first plurality and asecond plurality of driving electrodes, the display driver circuitrycomprising: a power supply connection; a plurality of electrode driveroutputs for driving a plurality of said display driving electrodes; aplurality of current driver circuits for each driving electrode forproviding a plurality of current drives for a said electrode driveroutput, each having a control input and each comprising a bipolarcurrent drive transistor having an emitter terminal substantiallydirectly connected to said power supply connection and a collectorterminal coupled to a said electrode driver output; and controlcircuitry coupled to the control inputs of said driver circuits andconfigured to provide a signal to each said control input forcontrolling the current drive provided to a said electrode driveroutput; whereby the brightness of each pixel is adjustable.
 21. A methodof increasing the efficiency of a current driver for an organicelectroluminescent display, the method comprising: providing a currentdrive for the display using a bipolar transistor with an emitterterminal substantially directly connected to a power line; and using avoltage for the power line such that when the organic electroluminescentdisplay is at substantially a maximum desired brightness the bipolartransistor current drive is operating substantially at-its limit ofcompliance.
 22. A method as claimed in claim 21 wherein the organicelectroluminescent display is a variable brightness display and saidcurrent drive is a variable current drive.